Derivative
The slope of a function at each point: the limit of (f(x + h) − f(x)) / h as h → 0, written f′(x).
The slope the secants approach is the derivative, written f′(x): f′(x) = the limit of (f(x + h) − f(x)) / h as h → 0 We can’t simply set h = 0, because that gives 0 / 0.
See it in action
Try derivative hands-on in the lesson Derivatives, part of the The maths behind AI course.